1. Field of the Invention
The present invention concerns a control method for coils of a transmission array and a gradient magnet system of a magnetic resonance system of the type wherein a control device of the magnetic resonance system activates the coils of the transmission array and the gradient magnet system corresponding to a respective excitation pulse, and wherein a magnetization that exhibits a first actual inhomogeneity is generated in an excitation volume of the magnetic resonance system due to the activation of the coils of the transmission array and the gradient magnet system corresponding to the respective excitation pulse.
The present invention furthermore concerns a computer readable medium encoded with machine code (programming instructions) that can be directly executed by a control device of a magnetic resonance examination system, with the execution by the control device causing such a device executes such a control method. The present invention also concerns a data medium with such a computer program stored on the data medium.
The present invention also concerns a magnetic resonance system of the type having a basic field magnet, a gradient magnet system, a transmission array with a number of transmission coils and a control device, wherein the transmission coils can be activated by the control device so that a magnetization is generated in an excitation volume of the magnetic resonance system by means of the transmission coils.
2. Description of the Prior Art
In magnetic resonance systems with basic field strengths greater than approximately 3 T, significant eddy currents are induced in the patient body. As a result, the actual homogeneous magnetic field distribution of the whole-body coil alone becomes more or less inhomogeneous within the patient. In isolated cases, this inhomogeneity can lead to the situation that a reliable imaging in specific body regions is problematic.
In conventional magnetic resonance systems, the achievable field distribution is permanently predetermined by the activation of the antenna.
The desired homogeneous magnetization (=deflection of the spins) can ensue via what are known as 2D or 3D excitation pulses in which radio-frequency and gradient pulse forms are simultaneously modulated. The modulation must be re-determined for every patient from the knowledge of the radio-frequency field distribution. This method is used in practice, but very long transmission pulses (for example longer than 10 ms) result.
Analogous to the use of multi-channel systems upon reception, the use of multi-channel systems can result in an under-sampling of frequency space in the transmission mode. The missing information can in this case be acquired from the different field profiles of the coils. The procedure is known to those skilled in the art under the designation “TX-SENSE”.
The known 2D or 3D excitation pulses can be “accelerated”. The duration of the excitation pulses is thereby reduced to 1 ms to 4 ms, for example, such that they exhibit lengths that are usable for imaging. A problem with such excitation pulses is in that the SAR (=specific absorption rate) and the transmission power requirements are many times higher (factors of approximately 20 to 30 are typical) in comparison to conventional single-channel transmission systems, even for the unaccelerated TX-SENSE pulses, and moreover the SAR and the power requirement for such pulses increase approximately quadratically with the acceleration.
Approaches by means of which the SAR can be reduced are known from scientific papers (that are published in ISMRM 2007, Page 673 and Page 674, for example). However, the SAR still lies well above the SAR of the single-channel solution.
These factors are also discussed, for example, in “An Image Domain Approach for the Design of RF Pulses in Transmit SENSE” by W. A. Grissom et al., Proceedings International Society of Magnetic Resonance in Medicine 13 (2005), page 19; “Parallel Excitation with an Array of Transmit Coils” by Yudong Zho, published in Magnetic Resonance in Medicine, Volume 51 (2004), Pages 775 through 784; and “Parallel Excitation: Making SENSE of High-Field Body MRI”, by Yudong Zhu; and United States Patent Application Publication No. 2005/134267.